6 Axillary Block
In the axilla the cords are located medially, laterally, and posteriorly, corresponding to their names (Fig. 6.1).
The ulnar nerve, the cutaneous nerve of the arm, the medial nerve of the forearm, and also part of the median nerve arise from the medial cord.
After the musculocutaneous nerve has arisen from the lateral cord, it forms a lateral root that unites with the medial root of the medial cord to form the median nerve.
Deep axillary fascia and deep axillary space.
From where it passes through the posterior interscalene space as far as the axillary region, the entire brachial plexus is surrounded by a connective-tissue sheath, which is called the deep axillary fascia in the region of the axillary fossa. The space lying under this is called the deep axillary space, which extends proximally to the infraclavicular region up to the interscalene space and further medially.
Besides the nerves, this space also contains the blood vessels (axillary artery and vein; Fig. 6.4 and Fig. 6.5). There are connective-tissue septa within this so-called neurovascular sheath (Fig. 6.6). However, in the majority of people, they do not appear to hinder the uniform spread of local anesthetic, so that block of the entire brachial plexus is possible with a single injection in the axillary region.
Localization of the nerves and axillary block.
The musculocutaneous nerve and the axillary nerve leave the neurovascular sheath very far proximally (Fig. 6.7). The axillary nerve is included in the axillary block only in a few cases and the musculocutaneous nerve only if the technique extends very far proximally and a corresponding volume of local anesthetic is used. The radial nerve lies in the axillary region behind the axillary artery and thus, depending on the technique employed, represents the second “problem nerve” in axillary block in addition to the musculocutaneous nerve.
6.2 Perivascular Single-Injection Technique
The patient lies supine, the arm is abducted about 90°, and the elbow is flexed about 90° and externally rotated. The axillary artery, which can usually be palpated readily, acts as a landmark. The coracobrachialis muscle runs cranial to the axillary artery. The palpating fingers find the gap between the axillary artery and coracobrachialis somewhat distal to the axillary crease. The injection site is located where the lateral edge of pectoralis major crosses the axillary artery (Fig. 6.8).
Following intracutaneous local anesthetic infiltration, a prepuncture is made through the skin for better penetration of the needle used for the block. This needle should have a short bevel (Fig. 6.9) for optimal identification of the neurovascular sheath. The needle is inserted at an angle of about 30 to 45° parallel to the artery in the palpated gap (Fig. 6.8).
After a few millimeters, noticeable resistance is felt, which can be overcome with controlled pressure. Immediately after overcoming this resistance, the needle is lowered and advanced proximally as far as it will go in the neurovascular sheath. A nerve stimulator can now be used to check that the needle is in the correct position. Using small “wobbling movements” different nerves can often be stimulated (median nerve, ulnar nerve, radial nerve) (Fig. 6.10, Fig. 6.11, Fig. 6.12, Fig. 6.13).
The tip of the needle is occasionally behind the median nerve, so it can be helpful to withdraw the needle tip toward the skin (anteriorly) to obtain a response. In contrast to all other blocks, there is no correlation here between the amplitude of the stimulus and the success rate.
Single-injection technique: 5 to 7.5 cm long atraumatic needle with a blunt bevel; pencil-point tip is also possible.
Continuous technique: 18G indwelling needle with blunt stylet (e.g., 45° bevel) (Fig. 6.9). After successful placement, remove the stylet, advance a flexible catheter through the indwelling needle (Fig. 6.14), and remove the needle.
For children: 20G indwelling needle with solid steel stylet.
A response from the musculocutaneous nerve indicates that the needle is in the wrong position (runs in the coracobrachialis muscle after leaving the brachial plexus; see Fig. 6.3). As an alternative to the use of a nerve stimulator, the correct needle position can also be verified by inducing paresthesia using refrigerated isotonic saline. In terms of effectiveness, this method is similar to the use of a nerve stimulator (Rodriguez et al 1996, Aul 2000), but for patients it is associated with unpleasant paresthesia.
Paresthesia should not be produced deliberately with the needle because of the increased risk of nerve injury.
Axillary plexus anesthesia performed by this method is one of the few techniques that can also be performed without the use of a nerve stimulator and/or ultrasound. For this, an 18G needle with a solid steel stylet, 45° bevel, and rounded edges is helpful to allow the loss of resistance to be felt clearly.
Apart from cold paresthesia and/or a response through the nerve stimulator, the following criteria are regarded as evidence that the needle is in the correct position:
Clear loss of resistance
Smooth advancement of the needle
The most frequent mistakes in insertion are incorrect orientation (artery not located correctly) and too-deep insertion. The needle must not be advanced deeper (in posterior direction) beyond the point of loss of resistance. (Lower the needle and advance it tangentially according to the procedure in peripheral venipuncture.)
For a suitable indication, bilateral catheter placement can be combined and performed and used even for severe skin injuries (Neuburger et al 2007) or frostbite.
In order to better reach the radial nerve with perivascular axillary block, use of the same technique, accessing the axillary neurovascular sheath inferior to the axillary artery, has been described (Meier et al 2003). With the patient in the position described above, the incision site in the bicipital medial groove is 3 to 4 cm distal to the intersection of the long head of the triceps brachii muscle with the latissimus dorsi, inferior to the neurovascular sheath. The needle is directed toward the groove between the teres major and the origin of the long head of the triceps brachii muscles. Use of a nerve stimulator and/or ultrasound is recommended.
6.2.2 Perivascular Axillary Block of the Brachial Plexus Using Ultrasound
Linear transducer: 7.5 to 10 MHz
Needle: 5 to 7.5 cm
Ultrasound Visualization of the Brachial Plexus, Transpectoral in the Short Axis
The transducer is placed transpectorally (after puncture and insertion of the indwelling needle as described in Chapter 6.2) at the site (Fig. 6.16) where the tip of the needle is expected to be (Geiser et al 2011). The axillary artery, positioned in the center of the image (Fig. 6.16) is used for initial orientation. Cranial to the artery is the coracobrachialis muscle, which is shaped like a club in this section, the pectoralis major is anterior, and the subscapularis is posterior. The artery and plexus lie in the angle formed by these three muscles (Fig. 6.16). As in the infraclavicular brachial plexus block, the individual cords/nerves are often difficult to identify in this section before injection of the local anesthetic (Fig. 6.16), so that it is important to place the tip of the needle in relation to the axillary artery (Fig. 6.17).